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    <title>Document</title>
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<body>

    <script>
        var obj = {
            name: '张三',
            age: 20

        }

        function observe(obj) {
            for (var key in obj) {
                Object.defineProperty(obj, 'age', {
                    get: function () {
                        console.log('get');
                        // return obj[key];
                        return 12
                    },
                    set: function () {
                        if (obj[key] != newvalue) {
                            return obj[key] = newvalue;
                        }
                    },
                    configurable: true,
                    enumerable: true
                })
            }
        }
        observe(obj);
        console.log(obj);

        
        
    </script>
</body>

</html>